A scanning electron microscope is a device which obtains an observation image of a target specimen in such a manner that a specimen to be observed is two-dimensionally scanned with a primary electron beam within an X-Y plane, and a secondary electron which is generated in a scan position or a reflection electron which is backscattered is detected, thereby imaging an output signal from a detector. The secondary electron or the reflection electron is generated along a trajectory of the scanning with the primary electron beam. Therefore, when an X scanning direction and a Y scanning direction are not orthogonal to each other, an obtained image is, of course, distorted.
FIG. 1 illustrates a relationship between orthogonality of scanning lines with respect to a scanning area and distortion of an image. When the scanning area in an X direction and a Y direction is not appropriate, an image is observed as a distorted image as shown in FIG. 1. In an example illustrated in FIG. 1, an object for observation is a circular specimen. For example, when the scanning area is distorted into a rhombus, an image is observed as being distorted into an ellipse. When the scanning area is an appropriate square area, the circular object is correctly observed as a circular image.
The orthogonal degree between an X scanning line and a Y scanning line is generally called “orthogonality”. In an actual scanning electron microscope, the orthogonality is, required to be adjusted to the extent with which the X scanning line and the Y scanning line can be regarded as being orthogonal to each other.
In a conventional electron microscope, a lattice specimen which is manufactured independently from an electron microscope has been used, and adjustment has been performed by applying correction to a control circuit of a scanning deflector of the electron microscope so that an image of the lattice specimen is observed as being orthogonal on a display screen, assuming that the lattice specimen is orthogonal. For example, PTL 1 discloses an invention in which a SEM image is divided into a lattice, and a lattice sheet is stuck on a monitor to thereby adjust deflection distortion of respective divided areas by visual observation.
However, in the conventional technique, since there is manufacturing variation in the accuracy of a lattice specimen, and the adjustment is performed by visual observation, there has been a problem in that adjusted orthogonality may vary between electron microscopes. Further, since a correction value for the control circuit is manually input, there has been a problem of variation in the correction accuracy. In addition, when a single user uses a plurality of electron microscopes, there has sometimes been a problem caused by an apparent difference in orthogonality between the electron microscopes.